1. Field of the Invention
The present invention relates to ambient-temperature-stable, one-part curable epoxy adhesive compositions, a method of curing the adhesive, a joint made with the cured adhesive and a method of making the joint.
2. Description of Related Art
Ambient-temperature-stable, one-part curable epoxy adhesive compositions that have a long shelf life over a broad range of storage conditions have long been the goal of the manufacturers of such products. Typically, storage conditions for such products would include temperatures which may range from well below 0xc2x0 C. to 50xc2x0 C. or higher.
Previous storage-stable, one-part, curable epoxy resin systems generally comprise two or more reactive or reaction-producing components stored in an intimately admixed, unreactive or slowly reactive state which, ideally, react rapidly when subjected to a suitable stimulus, such as application of heat or mechanical shear. Various attempts have been made to prepare one-part systems by utilizing as curatives latent chemical compounds which are stable at room temperature but will produce reactive curative material when heated to an appropriate temperature. Similarly, reactive curing agents and/or catalysts have been encapsulated into microcapsules typically having walls made of a polymeric material that is thermoset or cross-linked and, thus, are unaffected by heating but instead rely on shear forces to release the curing agent and/or catalyst to initiate the curing of the epoxy system.
Curable epoxy systems which contain curatives encapsulated in microcapsules having shell walls comprised of thermoplastic materials are known but the amount of the thermoplastic material in these compositions is generally limited to that just sufficient to provide a barrier between the curative and the curable epoxy systems and generally such amounts will have little or no effect in changing the physical properties of the resultant cured epoxy resin.
Curable epoxy systems which contain non-encapsulated curatives and thermoplastic particles are also known but such systems are not as storage stable as would be desired.
The use of engineering thermoplastic particulate to improve impact tolerance and delaminating resistance in composites is also known in the art as is the use of core-shell materials for use as tougheners in curable polymer.
A need exists for an ambient-temperature-stable, one-part curable epoxy adhesive composition which has excellent storage-stability under any of a wide variety of storage conditions, yet is easily cured by application of heat to produce a cured epoxy resin having excellent physical properties.
The present invention provides an ambient-temperature-stable, one-part curable epoxy adhesive composition which has excellent storage-stability under any of a wide variety of storage conditions, yet is easily cured by application of heat to produce a cured epoxy resin having excellent physical properties, including in some compositions a reduced modulus which makes it prone to cohesive failure which is desired in some applications. Compositions of the invention are useful in structural adhesive applications, either alone or in conjunction with conventional fastening techniques such as welding and/or mechanical fastening. The curable composition may be formed into sheets which provide a convenient way of introducing the adhesive composition between structural members to make a bonded joint. Preferred cured compositions according to the invention will fail cohesively before failing adhesively when adhered to a substrate.
Specifically, the present invention provides a one-part curable epoxy composition comprising a mixture of
A. epoxy resin capable of being cured to a cured epoxy resin when exposed to an activated latent curative system;
B. a latent curative system in an amount sufficient to cure said epoxy resin comprising (a) at least one epoxy-resin-miscible first curative contained substantially as a core within a multiplicity of ambient-temperature-stable, impermeable microcapsules having capsule walls comprised of a thermoplastic polymeric material and (b) at least one epoxy resin latent second curative admixed uniformly within said curable epoxy resin; and
C. sufficient particulate thermoplastic polymeric material having a melt flow temperature that exceeds ambient temperature and the ability to be at least partially melt blended into the epoxy resin to at least regionally plasticize the cured epoxy resin, wherein up to all of the total weight of the particulate thermoplastic polymeric material may be provided by the thermoplastic polymeric material of the capsule walls.
A preferred first curative comprises a solid epoxy resin hardener contained substantially as a core within the capsule walls of the microcapsules, and most preferably the curative also comprises a latent accelerator as the second curative which is not contained within the capsule walls with the first curative but may be contained in separate thermoplastic microcapsules.
The first curatives for use in curable compositions of this invention preferably are relatively non-acidic curing agents such as acid anhydrides of carboxylic acids, compounds containing the hydrazine function (xe2x80x94COxe2x80x94NHxe2x80x94NH2) or an epoxide-curing derivative thereof, diaminediarylsulfones, and dicyandiamide compounds including analogs of dicyandiamide which are disclosed in Anderson et al. (U.S. Pat. No. 3,553,166), the disclosure of which is incorporated herein by reference.
Imidazoles or imidazole containing compounds, hereinafter termed xe2x80x9cimidazolesxe2x80x9d or xe2x80x9cimidazolates,xe2x80x9d are preferably present in the curable composition as the second curative together with the first curative in catalytic amounts, i.e., in amounts sufficient to catalyze the reaction between the epoxide resin, the hardeners, and other reactive materials with the epoxide resin on heating of the composition. Metal imidazolate curative materials and their derivatives are disclosed in Hill et al. (U.S. Pat. No. 3,792,016), the disclosure of which is incorporated by reference. Preferably, the amount of the imidazolate used is about 0.1 to about 10 weight percent, more preferably about 0.5 to about 3 weight percent based on the epoxide equivalent weight. Preferably, the imidazolate is a solid which is insoluble at the storage and processing temperatures in the epoxide resin to provide increased storage stability.
The imidazoles and imidazolates useful in the practice of the invention include imidazole compounds having a counter ion to balance the charge in the molecule. A suitable imidazolate is a metal imidazolate compound of the formula:
MLm 
wherein M is a metal selected from the group of Ag(I), Cu(I), Cu(II), Cd(II), Zn(II), Hg(II), Ni(II) and Co(II), and L is an imidazolate of the formula 
wherein R1, R2, and R3 are selected from a hydrogen atom, an alkyl radical or aryl radical and m is the valence of M.
An example of a suitable imidazole is diphenyl imidazole.
The most preferred metal imidazolate compound is a green colored copper (II) imidazolate made as described herein.
The compositions according to the invention include sufficient thermoplastic polymeric material in particulate form to improve physical properties of the cured composition. Additionally, the thermoplastic polymeric materials may be entirely provided by the thermoplastic material which provides capsule walls which enclose other reactive components of the epoxy resin. Materials that may be encapsulated by the thermoplastic material include an epoxy hardener and/or accelerator, and other reactive materials, such as catechol, that in the absence of encapsulation may prematurely cause the epoxy resin to harden.
The thermoplastic polymeric material from the capsule walls and, if present, the particulate material, is contained in the composition in an amount based on the total weight of the cured composition, such that, after melt blending the thermoplastic material with the composition during curing, the epoxy composition is at least partially regionally plasticized and toughened. Plasticization is readily apparent in that the cured epoxy composition has at least areas having a reduced modulus when compared to the composition without the thermoplastic material. The modulus reduction is preferably sufficient to cause the cured adhesive composition to fail cohesively rather than adhesively when it is adhered to a substrate. Additionally, in preferred compositions of the invention, the presence of the thermoplastic material in the composition may increase the strength of the bond to certain substrates as evidenced by increased peel adhesion values. Typical amounts of thermoplastic material may range from about 0.5% to about 30% by weight of thermoplastic polymeric material based on the total weight of the epoxy adhesive composition, preferably about 1% to about 20%, more preferably about 2% to about 15% by weight and most preferably about 2% to about 10% by weight.
The invention further provides a method of making a cured epoxy composition comprising the steps of
A. providing a one-part curable epoxy composition comprising (a) epoxy resin capable of being cured to a cured epoxy resin when exposed to an activated latent curative system; (b) a latent curative system in an amount sufficient to cure said epoxy resin comprising (i) at least one epoxy-resin-miscible first curative contained substantially as a core within a multiplicity of ambient-temperature-stable, impermeable microcapsules having capsule walls comprised of a thermoplastic polymeric material and (ii) at least one epoxy resin latent second curative admixed uniformly within said curable epoxy resin; and; (c) sufficient particulate thermoplastic polymeric material having a melt flow temperature that exceeds ambient temperature and the ability to be at least partially melt blended into the epoxy resin to at least regionally plasticize the cured epoxy resin, wherein up to all of the total weight of said particulate thermoplastic polymeric material may be provided by the thermoplastic polymeric material of said capsule walls; and
B. heating said mixture at least to the melt flow temperature of said polymeric material.
Additionally, the present invention provides a one-part curable epoxy composition comprising a mixture of
A. epoxy resin capable of being cured to a cured epoxy resin when exposed to an activated latent curative system; and
B. a latent curative system in an amount sufficient to cure the epoxy resin comprising (a) at least one epoxy-resin-miscible first curative contained substantially as a core within a multiplicity of ambient-temperature-stable, impermeable microcapsules having capsule walls comprised of a thermoplastic polymeric material and (b) at least one epoxy resin latent second curative admixed uniformly within said curable epoxy resin; wherein the thermoplastic capsule walls isolate the first curative from the second curative.
The compositions described above may be stored in bulk containers for use with conventional dispensing systems which may heat the composition to a flowable consistency. The dispensing system preferably has the ability to force a segment of the composition from a dispensing nozzle, or to form the composition into a sheet which may be cut into strips and rolled for convenient dispensing by purchasers of such rolls. Such dispensing devices are well known by those skilled in the art, as are the methods of forming sheets of such compositions and as are methods of cutting the sheets into strips and rolling the strips onto suitable cores to provide rolls.
The compositions according to the invention are especially suited for use in adhering abutting members together, either by use of the cured composition alone or together with other mechanical means of fastening such as welding, bolting, riveting, sheet metal screw fastening, etc. The joint typically includes two abutting members, one having a surface which is in contact with a surface on the other wherein a quantity of the composition of the invention may be interposed therebetween. A typical place where such joints are found is in the assembly of automotive parts into an automobile. A joint is formed by holding the members together in a desired position while heating the curable composition to the melt flow temperature of the polymeric material to cause the microcapsules to open and release the first curative into the reactive epoxy resin while simultaneously activating the heat activatable latent second curative such as the metal imidazolate latent catalyst. The resulting cured epoxy resin will be characterized by including melt blended therein the thermoplastic material including that which formerly provided the capsule walls to thereby provide changes in physical properties in the cured epoxy resin. The changed physical properties of the cured epoxy resin include a multitude of plasticized particulate/epoxy resin interfaces or plasticized domains with a lower modulus than the non-plasticized or less plasticized domains, as compared to the same cured composition which does not have the thermoplastic material added solid particulate or capsule walls discretely contained therein. The curable adhesive of the invention is typically applied from a dispensing device to at least a portion of one surface of at least one member to be adhered to a surface of another member. Some heating may be required to provide a consistency of the curable adhesive such that it will flow under pressure. The adhesive is typically tacky and it will immediately adhere to the surface to which it is applied. The adhesive may also conveniently be applied as a sheet or strip.
The applied curable epoxy composition is quite useful in adhering together members for forming parts of automobiles during their production because it affords excellent flowability under pressure when the parts are being held together in a welding operation and may be welded through to provide a joint between such members which is both spot welded and adhered by the cured epoxy resin composition.
For the purposes of this invention, the following terms used herein will have the meanings designated below:
xe2x80x9cone-part curable epoxy compositionxe2x80x9d shall mean an integral composition which contains curable epoxy resin and other components at least one of which is a first curative which is encapsulated for release upon activation by heat and at least another of which is a heat-activatable latent second curative.
xe2x80x9cambient-temperature-stablexe2x80x9d with respect to the polymeric material forming the microcapsule wall shall mean that the microcapsules are stable under any of a wide variety of storage conditions which may range from xe2x88x9220xc2x0 C. or lower up to and including any elevated temperatures required for processing the curable composition, e.g., to form sheets.
xe2x80x9cimpermeablexe2x80x9d with respect to the microcapsule wall shall mean the capsule wall acts as a barrier between the curable epoxy resin and any curative contained therein under ambient temperature conditions.
xe2x80x9cmelt flow temperaturexe2x80x9d shall mean the temperature at which the thermoplastic polymeric material forming the capsule walls and the added particulate thermoplastic material first undergoes sufficient flow to permit interaction of the encapsulated curative and reactive epoxy resin to facilitate curing of the reactive epoxy resin and melt blending the thermoplastic particles and capsule walls into the epoxy resin.
xe2x80x9cmelt blendedxe2x80x9d with respect to the disposition of the thermoplastic polymeric material which formed the microcapsule walls, and that of any added particulate material, after achieving the melt flow temperature shall mean that such material will be distributed throughout the cured epoxy resin in a blend which may vary from being fairly homogeneous to being in isolated discrete phases within the cured epoxy resin.
xe2x80x9cmisciblexe2x80x9d shall mean with respect to the interaction of the first curative to the epoxy resin shall mean that the first curative at the melt flow temperature is sufficiently dispersed within the reactive epoxy resin to facilitate complete curing of the epoxy resin.
xe2x80x9csolidxe2x80x9d with respect to the physical condition of the epoxy resin hardener shall mean the hardener is a solid material, i.e., neither gaseous nor liquid under ambient temperature conditions.
xe2x80x9clatent curativexe2x80x9d shall mean an epoxy curative that has been rendered temporarily inactive so that it will not, under storage and processing conditions, cause curing of epoxy resin due to its being stabilized by chemical reaction to produce a chemical complex, being encapsulated within a material that provides a physical barrier (e.g., a thermoplastic polymer shell), or being inherently immiscibility and/or non-reactive.
xe2x80x9ccurativexe2x80x9d shall mean a compound that will initiate the curing of an epoxy resin, such as a hardener or accelerator.
xe2x80x9chardenerxe2x80x9d shall mean a curative having multiple functionality capable of cross linking the epoxy resin.
xe2x80x9cacceleratorxe2x80x9d shall mean a curative that promotes the rapid cure of the epoxy resin either alone or in combination with the hardener.
xe2x80x9cparticulate thermoplasticxe2x80x9d shall mean a solid thermoplastic particle that is not miscible at ambient temperatures in the reactive epoxy resin, which may take any shape such as a homogeneous sphere or microcapsule wall.
xe2x80x9cplasticizexe2x80x9d shall mean impart flexibility and toughness to at least a portion of the cured epoxy resin.
xe2x80x9cregionally plasticizexe2x80x9d shall mean to plasticize in zones around a site and/or former site of individual thermoplastic particles in a cured epoxy resin.